Air Rage?
Highly unlikely to be air rage. The hijacker must have demanded that the landing at Subang be aborted and proceed to Singapore. Somewhere along the flight to Singapore, things went wrong and the pilots were shot. Unfortunately the old CVRs only have the last 30 minutes of cockpit conversations remaining and most of the other pilot/hijacker interactions prior to the final 30 minutes were lost. I would surmise that Captain Ganjoor must have told the hijacker of the consequences expected once they land in Singapore, leading to the hijacker into thinking that it was a no win situation. Hence the rampage killing the pilots and then suicide.